The enzyme kynurenine aminotransferase (known in the art as KAT) catalyzes the biosynthesis of kynurenic acid (KYNA) from kynurenine (KYN) and is singularly responsible for the regulation of extracellular KYNA concentrations in the brain (J. Neurochem., 57:533-540 (1991)).
KYNA is an effective excitatory amino acid (EAA) receptor antagonist with a particularly high affinity to the glycine modulatory site of the N-methyl-D-aspartate (NMDA) receptor complex (J. Neurochem., 52:1319-1328 (1989)). As a naturally occurring brain metabolite (J. Neurochem., 51:177-180 (1988); and Brain Res., 454:164-169 (1988)), KYNA probably serves as a negative endogenous modulator of cerebral glutamatergic function (Ann. N.Y. Acad. Sci., 648:140-153 (1992)).
EAA receptors and in particular NMDA receptors are known to play a central role in the function of the mammalian brain (Watkins et al, In: The NMDA Receptor, page 242, (1989), Eds., Oxford University Press, Oxford). For example, NMDA receptor activation is essential for cognitive processes, such as, for example, learning and memory (Watkins et al, In: The NMDA Receptor, Eds., pages 137-151, (1989), Oxford University press, Oxford) and for brain development (Trends Pharmacol. Sci., 11:290-296 (1990)).
It follows that a reduction in NMDA receptor function will have detrimental consequences for brain physiology and, consequently, for the entire organism. For example, the decline in the number of NMDA receptors which occurs in the aged brain (Synapse, 6:343-388 (1990)) is likely associated with age-related disorders of cognitive functions.
In the brain, KYNA concentrations and the activity of KYNA's biosynthetic enzyme KAT show a remarkable increase with age (Brain Res., 558:1-5, (1992); and Neurosci. Lett., 94:145-150 (1988)). KAT inhibitors, by providing an increase of the glutamatergic tone at the NMDA receptor, could therefore be particularly useful in situations where NMDA receptor function is insufficient and/or KAT activity and KYNA levels are abnormally enhanced. Hence they could be particularly useful in the treatment of the pathological consequences associated with the aging processes in the brain which are, for example, cognitive disorders including, e.g., attention and memory deficits and vigilance impairments in the elderly.
KAT inhibitors may also be useful in the treatment of perinatal brain disorders which may be related to irregularities in the characteristic region specific pattern of postnatal KAT development (Baran et al, Dev. Brain Res., 74:283-286 (1993)).
In subcellular fractionation studies KAT activity was recovered in the cytosol and in mitochondria (J. Neurochem., supra).
Most nuclear-encoded precursors of mitochondrial proteins contain amino-terminal presequences (Pfanner et al, In: Current Topics in Bioenergetics, 15:177-219 (1987); Lee Ed., New York Academic Press; and Nicholson et al, In: Protein Transfer and Organelle Biogenesis, Das and Robins Eds., New York Academic Press (1988)). These presequences are required for the precursor to enter the mitochondrial matrix, where they are proteolytically removed (Hurt et al, FEBS Lett., 178:306 (1984); Horwich et al, EMBO J., 4:1129 (1985). This cleavage is not essential for completing import but is necessary for further assembly of the newly imported polypeptides into functional complexes (Zwizinski et al, J. Biol. Chem., 258:13340 (1983); Lewin et al, J. Biol. Chem., 258:6750 (1983); Ou et al, J. Biochem., 100:1287 (1986)). Precursor targeting sequences differ considerably in their structures. One of the few common themes is the high content of positively charged amino acids and of hydroxylated amino acids. Presequences may form an amphipathic structure in the form of either .alpha.-helices or .beta.-sheets (von Heijne et al, EMBO J., 5:1335 (1986); Roise et al, EMBO J., 5:1327 (1986); and Vassarotti et al, EMBO J., 6:705 (1987)). Despite the large variability of the sequences of mitochondrial leader peptides, relatively minor alterations of the presequence can prevent cleavage by the processing peptidase (Hurt et al, J. Biol. Chem., 262:1420 (1987)). This suggests that distinct, but up to now undefined, structural elements are required for cleavage. Similarly, the cleavage sites show wide variation among different precursors of a single organism and among precursors of different organisms.
Interestingly, using the protein algorithm described by Gavel et al (Protein Engineering, 4:33-37 (1990)), a potential mitochondrial transit peptide is predicted either in position 1 to 24 of the deduced protein of cDNA-2 and in position 1 to 44 of the deduced protein of cDNA-3 disclosed in the present invention (see FIGS. 3-4 and Example 3). Recently Perry et al (Mol. Pharm., 43:660-665 (1993)) reported the cloning of a cDNA coding for rat kidney cytosolic cysteine conjugate .beta.-lyase.. When the cDNA was inserted into the expression vector PVS1000 and transfected into COS-1 tissue culture cells, a 7-10 fold increase in cytosolic .beta.-lyase and glutamine transaminase K activities was detected. The deduced amino acid sequence of rat .beta.-lyase is identical to the deduced amino acid sequence of cDNA-1 (rat KAT) except for two residues (see FIG. 2). Moreover the existence of cDNA-2 and cDNA-3 was not reported by Perry et al (Mol. Pharm., supra).
Even more recently Perry et al (FEBS Lett., 360:277-280 (1995)) reported the cloning of a cDNA for human kidney cysteine conjugate beta-lyase whose sequence is identical to the sequence of the human KAT described in the present patent application. Whereas the identity with cysteine conjugate .beta.-lyase and glutamine transaminase K is well documented (Abraham et al, Analytical Biochem., 197:421-427 (1991)), there are no reports indicating identity of kynurenine transaminase with either .beta.-lyase or glutamine transaminase K.